Thermal processes, as their name implies, include all the processes in which heat is the driving force. Such processes include but are not limited to:
______________________________________ PROCESS EXAMPLES ______________________________________ combustion boilers, incinerators, furnaces calcination cement and ceramic calciners fusion or melting glass melters sintering ceramic kilns evaporation dryers ______________________________________
In most thermal processes, gaseous by products are exhausted from the process via an off-gas system. Gaseous byproducts can be simply water vapor in the case of, for example, a laundry dryer which is exhausted directly to the ambient air without posing any danger to the environment. Or, the gaseous byproducts could contain one or multiple toxic gases such as HCl, HF, UF.sub.6, NO.sub.x, SO.sub.x, CO, or toxic solid particulates, such as heavy metals, etc., which cannot be released to the environment without first being removed from the gaseous exhaust stream.
In U.S. Pat. No. 4,019,444 to Kleeberg et al., a method of using the heat of off-gas to evaporate scrubber solution resulting in dry waste is disclosed. The process uses an evaporator where slurry from a scrubber, after neutralization, is dried to complete dryness. The dried sludge particles are picked up in an electrostatic precipitator. The off-gas is then passed through a scrubber. The scrubber solution is allowed to settle in a clarifier. The supernatant is reused in the scrubber while the sludge is fed to a neutralizer. The disadvantage of the disclosure of Kleeberg is that the neutralizer increases the amount of solids contained in the system and the electrostatic precipitator is expensive. The powders accumulated in the precipitator are of low density and produce large volumes of waste.
Other patents relating to scrubber systems are U.S. Pat. No. 3,414,248 to Iwanaga et al., U.S. Pat. No. 3,332,884 to J. J. Kelmar, U.S. Pat. No. 3,803,804 to Arashi, et al., and U.S. Pat. No. 4,799,941 to Westermark.
In the patent to Kelmar a method of fixing radioactive fission product waste is disclosed. The system includes spraying a waste solution onto a burden of coal waste and combustible materials in a slagging furnace, and spraying scrubber solution onto the burden of coal. A problem is encountered in evaporating large quantities of water which require high energy for processing.